The present invention relates to a fast melting induction furnace which is provided with a pressing cover and which is operated at a high-frequency to fast-melt a cold material.
Japanese Patent Unexamined Publication Nos. Sho-55-118581 and Sho-61-168779 are known as conventional art literatures relating to a fast melting induction furnace provided with a pressing cover. In the literatures, the lower surface of the pressing cover is made plane. In a fast melting induction furnace, generally, if electric power is supplied at a high frequency to fast-melt a cold material, the material swells at its center portion. Therefore, the electric power cannot be supplied sufficiently in the devices disclosed in such publications. Also, such swell-of the material increases the surface area of molten metal so that both oxidation and gas absorption increase. In view of the above-noted difficulty, an up/down movable pressing cover is arranged in the inside of a crucible in order to suppress the swell of the molten metal and exclude the outside air.
FIG. 2 is a sectional view of such a conventional art. The swell of molten metal 3 in the inside of a crucible 2 on which coils 1 is wound is suppressed by a pressing cover 24 having a lower surface made plane. In order to elevationally move the pressing cover 24 within the inside of the crucible 2, a gap is inevitable formed between the pressing cover 24 and the crucible 2. As is well-known, the molten metal 3 makes a motion under electromagnetic force as expressed by the arrows in the drawing. This causes the swelling of molten metal. Because of the motion of molten metal and the existence of such a gap, there occurs blow-out 23 of molten metal 3 through the gap. The blow-out 23 is so dangerous that the supply of high electric power is prohibited to make fast-melting impossible.